Electronic tape measures with a digital display are already available and often include a calculator, mounted on a case incorporating a wound tape, for use as a normal calculator and/or for use in computing various information based on one or more distances measured by the tape measure.
The distance measured in all these devices requires some arrangement for determining the length of tape which has been drawn out of the case at any time as well as determining, as the tape is moved, whether the tape is being moved out of or in to the case. A digital display is provided for displaying, usually at all times, the amount, that is the length, of tape which is currently extending out of the case.
The most direct way of monitoring the tape is to "read" optical, magnetic or other markings on the tape itself. It has already been proposed to provide equally spaced perforations in the tape and to sense movement of the tape by exposing the tape to a light source on one side of the tape and sensing light that passes through the perforations as the tape moves past the sensor. Such arrangements cannot in themselves provide an indication of which direction the tape is moving and are particularly inaccurate if the tape judders, sensitivity of the sensor varies or the intensity of the light source changes, and so on.
Further, the resolution of such arrangements cannot be very high without significantly weakening the strength of the tape, that is, other than by having very many perforations per unit length.